The present invention is directed to multicomponent pressure sensitive adhesives based on the use of block copolymers containing ethylene-olefin blocks and particularly the equivalent of a saturated ethylene-butylene or ethylene-propylene blocks copolymerized with polystyrene endblocks as modifiers for tackified ethylene-propylene rubbers.
Ethylene-propylene rubbers have been known in the art to provide curable pressure sensitive adhesive compositions which can be cast out of solvent. Their acceptance in the art, however, has been limited as there exists quite stable acrylic based copolymers with saturated or unsaturated backbones which also provide excellent elevated temperature properties. Acrylic based pressure sensitive adhesives, usually require the use of a solvent or water as the carrier, where the adhesive polymer is emulsifiable, to enable proper casting onto a substrate. This requires the use of a solvent recovery system or means to eliminate water as the carrier.
A more desirable method of casting an adhesive is from a hot melt. In this method of application, a pressure sensitive adhesive composition is applied to a substrate such as paper or Mylar.TM., solvent and carrier free, by melting and coating the adhesive composition at some temperature below a degradation temperature at which it will flow and which when cooled to a use temperature, normally ambient, will solidify to a tacky end product.
Pressure sensitive adhesives have been formulated using natural rubber, linear and multi-armed block copolymers comprising isoprene or butadiene such as polyisoprene, styrene-butadiene (SB) and styrene-butadiene-styrene (SBS), and mixtures thereof, styrene-isoprene (SI) and styrene-isoprene-styrene (SIS) block copolymers and mixtures thereof and the like.
While SIS and SBS block copolymers are widely used, they show inferior aging characteristics due to the presence of high levels of unsaturation in the backbone.
Hydrogenated block copolymer such as styrene ethylene-butylene (SEBS) based pressure sensitive adhesives exhibit excellent adhesion compared to acrylics, and excellent aging compared to styrene-isoprene and styrene-butadiene block copolymer based adhesives. However, the high temperature performance of such adhesives has been poor compared to solvent based acrylic adhesives.
U.S. Pat. No. 4,152,231 discloses that high temperature performance of styrene-isoprene and styrene butylene based adhesives could be enhanced using radiation cure in presence of multifunctional acrylates while U.S. Pat. No. 4,820,746 discloses means to improve high temperature performance while maintaining high peel adhesion.
U.S. Pat. No. 4,578,429 describes a thermally stable, modified, selectively hydrogenated, 1,2 diene block copolymer, wherein a maleic anhydride functional group is grafted on to the polymer backbone. The product is described as useful in providing an interaction not possible with pure hydrocarbon polymers, and in binding proteins such as heparin for blood compatibility.
European Patent Publication 1 780 62 discloses a pressure sensitive adhesive based on ethylene-propylene thermoplastic elastomer comprising crystalline polypropylene sequences and amorphous ethylene-propylene sequences along the chain. Such polymers are claimed to have good balance of peel, shear and tack properties.
European Patent Publication 2 540 02 disclosed a pressure sensitive adhesive with enhanced tack comprising an elastomer, tackifier and low molecular weight ethylene-propylene copolymer obtained by oxidative degradation.
U.S. Pat. No. 4,756,337 discloses a tackified ethylene-propylene based pressure sensitive adhesive tape for sealing gas pipe lines.
Ethylene-propylene rubber (EPR) based pressure sensitive adhesives are known although not widely used in the adhesives industry. Such adhesives have normally only been shown to have excellent adhesion characteristics when crosslinked by radiation or chemical crosslinking. When uncrosslinked, EPR based adhesives showed inferior room temperature shear properties compared to their cured counterparts.
This invention pertains to improving the shear adhesion properties of tackified EPR polymers by incorporating modifying ethylene-propylene and ethylene-butylene block copolymers.